The present invention broadly relates to composite bearing materials which are comprised of a hard metal backing strip, such as steel, having a bearing lining composed of leaded bronze tenaciously bonded to at least one face surface thereof. Such composite bearing materials are eminently suitable and in widespread use for the fabrication of various bearing components for use in internal combustion engines, vehicle suspensions, transmission assemblies or the like.
Composite bearing materials of the foregoing general type have been produced by processes such as disclosed in U.S. Pat. No. 2,986,464 granted May 30, 1961 to Lewis et al and U.S. Pat. No. 4,002,472 granted Jan. 11, 1977 to LeBrasse et al which are also assigned to the assignee of the present invention. The teachings of the two aforementioned U.S. patents, to the extent that they are relevant to the present invention, are incorporated herein by reference.
While the processes disclosed and the resultant composite bearing material produced in accordance with the processes described in the aforementioned U.S. patents are eminently suitable for producing high quality composite bearing materials for the fabrication of various bearing components, less than optimum physical properties of the bearing lining and performance of the bearing components produced therefrom have been obtained due to the presence of relatively large-sized lead particles in the bearing lining. This is particularly true where, because of engine operating conditions (e.g. high dynamic loads, acidic engine oils and increased oil temperatures) a electrodeposited lead tin or lead tin copper overplate of the bearing material is required or desirable. This is because of the well known diffusion phenomena as described more fully in SAE Technical Paper 860355, authored by one of the coinventors of the present invention, the teachings of which are incorporated herein by reference to the extent relevant to an understanding of the present invention. At engine temperatures random diffusion of the tin atoms in the lead tin or lead tin copper overplate results in the formation of a layer of nickel tin intermetallic compound on top of the nickel barrier. The function of the nickel barrier is to prevent diffusion of the tin into the underlying copper lead. In the absence of a nickel barrier extensive tin diffusion takes place via the continuous lead phase, copper tin compound forms at the copper-lead interface and the loss of tin is much more serious. The tin content necessary to provide resistance to corrosion by acidic engine oils is around 3 percent. If there is no nickel barrier the tin content will fall to this value more quickly than when a nickel barrier is present, and the loss of tin is restricted to the formation of nickel tin compound only.
Under the high dynamic loads applied, particularly to the connecting rod bearings of the heavy duty diesel engine, breaks in the nickel barrier can occur. The breaks are found above the lead phase and result in a path being made available for diffusion of the tin atoms through the nickel barrier into the copper lead. Because the tin atoms are trapped in the copper lead as the copper tin compound, lead is forced out of the copper lead, carrying the broken nickel barrier with it. The breaks widen, permitting more tin diffusion and the broken section of the nickel barrier may end up half way through the thickness of the overplate. The likelihood of a nickel barrier break occurring is a function of the size of the lead phase underneath it. The coarser the lead the less the support for the barrier and the more likely a break is to occur.
Briefly stated, for this reason, attention has been given to factors affecting the lead size in sintered copper lead alloys, and process changes have been introduced which restrict growth of the lead during sintering and minimize the nickel barrier breakage during engine service. The present invention provides for an improved process and an improved composite bearing material produced thereby employing powder metallurgical techniques whereby a satisfactory tenacious bond is obtained between the bearing lining and the steel backing strip employing sintering conditions including time and temperature which inhibit the formation of large-sized lead particles thereby achieving a unique leaded-bronze lining characterized by an extremely fine-sized lead distribution dispersed uniformly throughout the bearing lining matrix.